Abstract

Introduction

In breast cancer, deregulation of the WNT signaling pathway occurs by autocrine mechanisms.
WNT ligands and Frizzled receptors are coexpressed in primary breast tumors and cancer
cell lines. Moreover, many breast tumors show hypermethylation of the secreted Frizzled-related
protein 1 (sFRP1) promoter region, causing low expression of this WNT antagonist.
We have previously shown that the WNT pathway influences proliferation of breast cancer
cell lines via activation of canonical signaling and epidermal growth factor receptor
transactivation, and that interference with WNT signaling reduces proliferation. Here
we examine the role of WNT signaling in breast tumor cell migration and on xenograft
outgrowth.

Methods

The breast cancer cell line MDA-MB-231 was used to study WNT signaling. We examined
the effects of activating or blocking the WNT pathway on cell motility by treatment
with WNT ligands or by ectopic sFPR1 expression, respectively. The ability of sFRP1-expressing
MDA-MB-231 cells to grow as xenografts was also tested. Microarray analyses were carried
out to identify targets with roles in MDA-MB-231/sFRP1 tumor growth inhibition.

Results

We show that WNT stimulates the migratory ability of MDA-MB-231 cells. Furthermore,
ectopic expression of sFRP1 in MDA-MB-231 cells blocks canonical WNT signaling and
decreases their migratory potential. Moreover, the ability of MDA-MB-231/sFRP1-expressing
cells to grow as xenografts in mammary glands and to form lung metastases is dramatically
impaired. Microarray analyses led to the identification of two genes, CCND1 and CDKN1A, whose expression level is selectively altered in vivo in sFRP1-expressing tumors. The encoded proteins cyclin D1 and p21Cip1 were downregulated and upregulated, respectively, in sFRP1-expressing tumors, suggesting
that they are downstream mediators of WNT signaling.